Added transmission parsing of XML/URDF files

This creates the data structures and parser to follow a transmission format outlined in the following two examples:

**Simple transmission**

```xml
<transmission name="simple_trans">
  <type>transmission_interface/SimpleTransmission</type>
  <joint name="foo_joint"/>
  <actuator name="foo_motor">
    <hardwareInterface>EffortJointInterface</hardwareInterface>
    <mechanicalReduction>50</mechanicalReduction>
  </actuator>
</transmission>
```

**Finger**
```xml
<transmission name="finger_trans">
  <type>transmission_interface/FingerTransmission</type>
  <joint name="f1_joint">
    <role>proximal_knuckle</role>
    <hardwareInterface>EffortJointInterface</hardwareInterface>
    <mechanicalReduction>3</mechanicalReduction>
  </joint>
  <joint name="f2_joint">
    <role>distal_knuckle</role>
    <hardwareInterface>EffortJointInterface</hardwareInterface>
    <mechanicalReduction>2</mechanicalReduction>
  </joint>
  <actuator name="finger_motor">
    <hardwareInterface>EffortJointInterface</hardwareInterface>
    <mechanicalReduction>50</mechanicalReduction>
  </actuator>
</transmission>
```

There's one TODO that needs to be done asap:
```
# TODO: add tranmission parser to ROSBUILD - I don't know how to use rosbuild
```